This invention relates to a material constituting an optical element used in the fields of optical communication, optical measurement and laser engineering.
Recent years have seen remarkable development of optical technology with the advanced optical communications systems and advanced lasers. The demands for precision and performance of optical elements used in these fields have been increasing accordingly. Glass is one of the most important materials of optical components such as optical fibers and optical lenses. Glass can have a wide variety of compositions which can be selected in conformity with the application. Glass for such use is required to have stability as well as optical transparency. While glass is usually relatively excellent in weather resistance and heat resistance, some compositions have poor water resistance or insufficient thermal properties for particular applications.
Included in characteristics required of optical materials is stability against temperature. Stability against temperature means unchangeability of the characteristics with temperature changes. That is, heat-resistant glass does not always have stable characteristics against change in temperature.
When temperature changes, an optical material changes in not only refractive index but length, and the two changes cooperatively result in a change of optical path length. According to Izumiya Tetsuro, Kogaku Glass (Optical Glass), Kyoritsu Syuppan (1984), the relationship between thermal expansion coefficient xcex1a and temperature T dependence of refractive index na is established by equation (1) with the optical path length of the space that changes with thermal expansion being taken into consideration:
dna/dT+(na1xe2x88x92)xcex1a=0xe2x80x83xe2x80x83(1)
Where a material having instable optical characteristics against temperature is used as an optical element, for example, a medium of a laser, the beam mode would change. Where applied to a prism for optical path length control in an interferometer, the optical path length would be changed delicately. According to Kogaku Glass, the temperature coefficient of optical path length s, ds/dT, is 6xc3x9710xe2x88x926/xc2x0 C. in the case of LSG91H glass. Such a temperature-dependent change in optical path length causes considerable instability particularly in an interferometer. A material called athermal glass has been developed to eliminate the above-described drawback, whose optical path length change with temperature is near to zero.
Since it is necessary to select constituting components for athermal glass so as to satisfy equation (1), athermal glass eventually contains increased amounts of phosphoric acid, boric acid, and the like and therefore has poor water resistance, which is problematical for broad applications. Further, the composition being limited, it is difficult to control the physical properties characteristic of glass, such as refractive index.
Furthermore, optical elements have recently shown marked development in function with the development of optical communication. In particular, an optical wavelength division multiplex (WDM) communications system has gained in importance to cope with the growing demand for communication capacity. Unlike a conventional single-channel optical communications system which transmits one wavelength, e.g., 1550 nm, per optical fiber, the WDM system transmits different wavelengths simultaneously to increase the transmission capacity. An 8-channel system and a 16-channel system having the peak wavelengths equally spaced by 1.6 nm or 0.8 nm, respectively, have been put into practical use. There is a tendency that the number of channels increases, i.e., the spacing between wavelengths decreases. In the WDM system, such functions as combining (multiplexing) optical signals of different wavelengths or dividing (demultiplexing) optical signals into different wavelengths are of importance. For dividing wavelengths, the diffraction effect or interference effect of light has been utilized.
An optical fiber Bragg grating is a representative element for wavelength division. As shown in FIG. 4, an optical fiber having a clad layer 14 and a core 13 containing SiO2 and GeO2 is irradiated with ultraviolet light 15 having a periodic intensity distribution (e.g., an excimer laser beam having a wavelength of 248 nm) to form high-refractive index portions 19 and low-refractive index portions periodically in the core 13, which function as an optical fiber Bragg grating 11. A periodic light intensity distribution can be developed by a phase mask 16, etc., and diffracted light beams 17 and 18 are made to interfere with each other. The optical fiber Bragg grating 11 takes an important role in the WDM communications system as a demultiplexing element, serving to reflect and isolate only a desired wavelength.
Such a Bragg grating can be formed in not only an optical fiber but a flat member, such as a photosensitive polymer film, etc. by periodically forming high-refractive index portions and low-refractive index portions, which can be applied as a demultiplexing element or filter.
Further, systems utilizing the interference effect of light beams passing through two-split optical paths are also used in optical communication technology, such as a Mach-Zehnder interference element of optical fiber type as described in J. Lightwave Technology, vol. 16, p. 265 (1998) and a Mach-Zehnder interference element of optical waveguide type as described in ibid, vol. 17, p. 771 (1999).
These optical elements change in refractive index and physical length with temperature changes. These two changes cooperatively result in a change of optical length, which will cause, for example, the above-described optical fiber Bragg diffraction grating to shift its reflection wavelength from a set value. For instance, J. Lightwave Technology, vol. 14, p. 58 (1996) reports that the wavelength shift in a silica optical fiber due to temperature change is about 0.01 nm/xc2x0 C. That is, if temperature changes from xe2x88x9220xc2x0 C. to 80xc2x0 C., the reflection wavelength is shifted to the longer wavelength side by about 1 nm. Such a shift width exceeds the wavelength spacing in the above-mentioned WDM communications system, which is a serious problem making the system unworkable. The optical fiber Bragg diffraction grating is a single example, and a similar problem arises in other optical elements.
An object of the invention is to provide an optical element which shows suppressed change in optical length with a temperature change and is therefore applicable to a variety of optical systems including optical WDM communication.
The above-described problems are solved by making up the optical path material of an optical element of a mixture or a composite of a first material and a second material, the temperature coefficients of refractive index of the first and second materials having opposite signs. (That is, if the temperature coefficient of refractive index of the first material is positive, that of the second material is negative, and vice versa.) The material includes a mixture or a composite of an organic material and an inorganic material.
In the present invention, the optical element may comprise a solid material and a space comprising one of air and vacuum in the optical path thereof (a first embodiment), or may consist essentially of solid (a second embodiment). In the second embodiment, for example, the function of reflecting a light having a specific wavelength selectively is completed in the inside of the element (e.g., waveguide type diffraction grating). In the first embodiment, the optical element may be used for a medium for a laser, a lens, a prism or a beam splitter. In the second embodiment, the optical element may be used for a diffraction grating (waveguide type), an interference filter, a photonic crystal, an etalon or an optical waveguide.